(1) Field of the Invention
The present invention relates to a revolution frequency indication apparatus which indicates the revolution frequency of a rotating body by means of a revolution sensor mounted on said rotating body or its surrounding, and indicates a measured result.
(2) Description of the Prior Art
With a conventional tachometer namely a speed indication apparatus known in the prior art, the revolution frequency of a rotating body is measured while a measured result being indicated on an indicator. At that time, revolution sensors are mounted on, for example, said rotating body or its surrounding and generate the same number of pulses as the number of said revolution sensors when said rotating body revolves by one turn. Thereby, the revolution frequency of the rotating body is measured and indicated according to the time intervals of generated pulses.
FIG. 5 shows the timing of output pulses from a revolution sensor and the indication of a revolution frequency with a conventional revolution frequency indication apparatus known in the prior art. As shown in FIG. 5, a time to an input pulse (time interval of pulses) is measured while defining and indicating a revolution frequency of the rotating body according to a measured time interval.
With such a conventional revolution frequency indication apparatus as described above and known in the prior art, however, there was a problem that indicated values of revolution frequencies often varied so quickly when the frequencies were high that a supervisor could not ascertain what the frequencies really were, because the time of updating the indication depended on the duration of a sensor pulse (pulse time) generated by the revolution sensor. There was also another problem with such an apparatus known in the prior art that a zero ("0") frequency could not be indicated when the rotating body suddenly stopped, because each measurement cycle could be completed only after two pulses were entered.